The cleaning of integrated circuit (IC) substrates, such as silicon wafers, with metal-free alkaline solutions to remove organic and metal contamination is widely practiced. One commonly used alkaline solution of this type is known as SC-1 or RCA-1 and comprises a hot aqueous mixture of ammonium hydroxide, hydrogen peroxide, and water (1:1:5 of 30% H.sub.2 O.sub.2, 28% NH.sub.4 OH and H.sub.2 O) to remove organic impurities and copper contamination from a wafer surface. Various cleaning tasks can be accomplished with SC-1, among these, the cleaning of silicon wafers immediately after their fabrication, the cleaning of such wafers immediately prior to gate oxide growth, the removal of oxide etch residues later in the IC processing sequence, and selective etching and resist particulate removal.
Treatment of the wafer surfaces with the hot SC-1 or RCA-1 solution is generally followed by a hot acid solution known as SC-2 or RCA-2 to remove metals untouched by the SC-1 or RCA-1 solution. This hot acid solution SC-2 comprises hydrogen peroxide, hydrochloric acid and water (1:1:5 of 30% H.sub.2 O.sub.2, 37% HCl and H.sub.2 O).
Both solutions, SC-1 and SC-2 contain hydrogen peroxide. The purpose of the hydrogen peroxide is to protect the silicon metal from exposure to strong acids or bases by continuously forming a protective oxide layer in order to prevent etching or roughening of the silicon surface.
It is, however, necessary for the wafer surfaces to be oxide-free to be suitable for further processing where an oxide surface is not wanted. Usually, it is then necessary to remove the protective oxide layer formed by the hydrogen peroxide in the cleaning solutions. As an example of a material commonly used to remove such protective oxide layer, there may be mentioned HF.
The presence of hydrogen peroxide in the formulations imparts an inherent instability to these solutions. Such solutions typically exhibit peroxide half-lives of less than one hour at 70.degree. C. The hydrogen peroxide in the SC-1 solution in the presence of certain metals, particularly copper and iron, becomes unstable and decomposes in rapid exothermic fashion leading to potentially dangerous conditions. The hydrogen peroxide has a low tolerance for metal contamination. Additionally, the decomposed hydrogen peroxide drops the concentration of the hydrogen peroxide leading to the possibility of silicon etching producing wafers that are not acceptable for IC manufacture. Thus, the decomposed hydrogen peroxide needs to be replenished and this changes the solution composition thereby varying the cleaning properties of the solution. In addition, the inherently high pH of the hydrogen peroxide solution presents undesirable safety and environmental concerns.
Since the introduction of the SC-1 or RCA-1 solution, there have been proposals for using basic materials other than ammonium hydroxide to clean wafer surfaces. For example, quaternary ammonium hydroxide compounds, such as tetramethyl-ammonium hydroxide (TMAH) or trimethyl-2-hydroxyethyl ammonium hydroxide (choline) have been reported in Japanese Patent Publications No. 3-93229 and 63-114132; U.S. Pat. Nos. 4,239,661; 4,964,919 and 5,259,888 and European Patent Publication No. 496605, for example. It is to be noted that the wafer roughness values mentioned in U.S. Pat. No. 4,964,919 are unacceptable for high density integrated circuit manufacture. Moreover, U.S. Pat. No. 5,207,866 describes a case where a quaternary amine without hydrogen peroxide present is used to anisotropically etch the silicon 100 face of wafers.
Without hydrogen peroxide present, none of the above mentioned alkaline or quaternary ammonium hydroxide-based cleaners can produce the wafer smoothness levels necessary for high density integrated circuit manufacture. Recently two technologies have been disclosed that permit cleaning without the use of hydrogen peroxide while maintaining acceptable roughness levels. In U.S. Pat. No. 5,466,389, the cleaning compositions contain a nonionic surfactant and a component to reduce or control the pH within the range of about pH 8 to about pH 10. In U.S. Pat. No. 5,498,293, the cleaning compositions contain an amphoteric surfactant. In both cases, wafer smoothness is maintained without the use of hydrogen peroxide.
While these new technologies can be used to clean wafer substrates without the use of hydrogen peroxide, both methods involve the introduction of organic surfactants to the cleaner formulation. These organic components could ultimately be absorbed onto or left on the wafer surface as residual matter. Organic contamination is a serious issue in the manufacture of a semiconductor device. The presence of organic contaminants on the surface of a silicon wafer can lead to the formation of silicon carbide when a thermal treatment, such as the growth of a thermal oxide, is carried out on a wafer. Silicon carbide may then be incorporated into the crystal substrate and cause defects in the crystal lattice. These crystal defects act as carrier (electron) traps that cause premature breakdown of the gate oxide and therefore cause the failure of the semiconductor device. Inorganic contaminates can also be deposited along with the organic contaminates on the surface, which also leads to the premature breakdown of the dielectric gate oxide. Organic contamination also prevents the removal of any underlying native oxide. This leads to incomplete oxide removal during a subsequent treatment to remove the oxide and would lead to an increase in microroughness and uneven gate oxide regrowth. Any increase in microroughness causes an uneven interface to result when a thin oxide or some other layer is formed in contact with the substrate and may result in decreased film integrity. Deviations in the thickness of these layers can seriously affect device performance or even lead to the failure of the device. Other negative effects associated with organic contamination that have been reported are; unintended hydrophobization, increased deposition of particles, unintended counterdoping, prevention of silicon wafer bonding, prevention of classical bonding, decreased metal pad adhesion, corrosion, chemical carryover, and image formation on wafers.
Several methods have been used to remove such residual organic contamination. One method uses ozonized ultra-pure water but this involves additional steps and requires special equipment to generate the ozonized water (S. Yasui, et. al., Semiconductor Pure Water and Chemicals Conference Proceedings, pp 64-74, 1994). Clearly, it would be advantageous to avoid use of organic surfactants during the initial "front end" cleaning of semiconductor wafer surfaces.
Surfactants and other alkaline organic solutions containing alkane diols have been used for stripping photoresists in the past. Photoresist stripping involves the removal of various residues from metal or dielectric integrated circuit elements. In U.S. Pat. No. 4,744,834 (N-methylpyrrolidone derivative or glycol ether required), U.S. Pat. No. 5,091,103 (N-methylpyrrolidone required), U.S. Pat. No. 4,770,713 (amide solvent required), and U.S. Pat. No. 5,139,607 (cosolvents required), various additional solvents are required to produce the desired stripping action. In the case involving cleaning of silicon wafers, the potential organic contamination by these cosolvents would be highly undesirable.
Surfactants and other organics are used in strippers and cleaners designed to remove photoresist from wafers. Photoresist is used in generating patterned metal features needed in a functional integrated circuit (IC) and is considered to be part of the "back end" processing of the wafer. Since photoresist is a polymeric organic material, it is apparent that organic contamination is less critical at this stage in the processing of the IC.
Photoresist stripping almost always involves contacting a corrosion sensitive metal circuit component with the stripper. For this reason the water content of photoresist strippers is kept to a minimum (less than 20%) to avoid corrosion. In the glycol containing formulations described in U.S. Pat. No. 4,765,844 and U.S. Pat. No. 5,102,777, no water is specified.
Several stripper formulations that have been disclosed (U.S. Pat. No. 5,482,566 , U.S. Pat. No. 5,279,771 , U.S. Pat. No. 5,381,807 , and U.S. Pat. No. 5,334,332) that require the presence of hydroxylamine. This component is included to reduce the corrosive action of the highly alkaline formulations that are claimed. The use of strongly reducing media for this purpose has been published (Schwartzkopf, et. al., EP Patent Application 647,884, Apr. 12, 1995). The use of hydroxylamine for cleaning wafer substrates would be detrimental since the highly reducing medium would convert the metal impurities D to less soluble reduced forms which may in turn be deposited onto the silicon surface as elemental metals.
It is an object of this invention to provide a cleaning solution for cleaning wafer substrates of metal contamination without increasing surface microroughness, which cleaner composition does not require the use of hydrogen peroxide to provide a protective oxide layer, or the use of organic surfactants. A further object of this invention is to provide a cleaner composition for cleaning wafer substrates of metal contamination without increasing surface microroughness and leaving an essentially oxide-free wafer surface, making the surface suitable for further processing where an oxide surface is not wanted. A still further object of this invention is to clean such wafer surfaces of metal contamination without requiring an acid treatment step or the use of materials, such as HF, used to remove oxide surfaces. An additional aspect of this invention is to provide a process for cleaning such wafer surfaces of metal contamination by using only a single cleaning solution without increasing wafer surface microroughness. Yet another object of this invention is to provide a process and composition for cleaning such wafer surfaces of metal contamination without increasing wafer surface microroughness using an aqueous alkaline solution, and more particularly, using an aqueous quaternary ammonium hydroxide solution free of both hydrogen peroxide or other oxidizing agents and organic surfactants. Yet another object of this invention is to provide such a process and alkaline cleaning composition for cleaning wafers and producing a roughness of less than about 25 Angstroms as the average distance in the Z direction between wafer peak heights and valleys.